1. Field of the Invention
The present invention relates to a method and apparatus for the continuous automatic analysis of the crystallization point of liquid substances, especially of aromatic hydrocarbons.
2. Description of the Prior Art
A method and an apparatus for the automatic continuous analysis of the filterability point of liquid substances, in particular of gas-oils during manufacture, is described in U.S. patent application Ser. No. 505,645 filed by applicant on Sept. 13, 1974.
The method consists of passing a substance to be analyzed through a measuring circuit at a constant flow rate, the temperature therein being maintained at a value sufficiently below the assumed filterability point of the substance, heating means being provided along the flow path of the liquid substance in said circuit, and automatically turning on and off said heating means respectively at two characteristic temperatures detected by measuring the pressure drop or difference in pressure of the substance at the inlet and at the outlet of the circuit, the variation of said pressure drop being effected as a function of temperature, the values thus measured of the pressure drop being utilized as control data for respectively turning on and off the heating means, when the temperature at the outlet of the substance reaches one of the two characteristic temperatures, the first corresponding to a temperature slightly below the filterability point, whilst the other corresponds to a temperature sufficiently above said filterability point, and in that the temperature of the substance at the outlet of the circuit is recorded continuously, which temperature constitutes a representative value of the filterability point of the substance.
A preferred embodiment of the apparatus for carrying out the aforementioned method comprises a measuring circuit constituted by a capillary tube of stainless steel traversed by the substance to be analyzed, whose flow rate is kept constant by a small volumetric pump, the capillary tube having a large portion of its length immersed in a cooling liquid bath contained in a measuring well, the inlet and the outlet of the capillary tube in the well being connected, outside of the well, to two electrical leads of which one is directly connected to the power supply whilst the other is connected to differential pressure-responsive switching means, mounted between the inlet and the outlet of the capillary tube, whilst a thermosensitive element is positioned at the outlet of the measuring well along the flow path of the substance to be analysed and is connected to a recording device.
The aforementioned patent application also describes a method and an apparatus for the continuous measurement of the filterability temperature limit, the filterability point corresponding to the appreciable diminution of the flowability of a gas-oil when the temperature drops.
Devices have also already been described, enabling the determination of the crystallization temperature of a liquid. Reference may notably be made to U.S. Pat. No. 3,577,765, which relates to a method and an apparatus for determining automatically the crystallization temperature of a flowing liquid and more particularly of aqueous solutions, for example of sodium benzenesulfonate whose crystallization is not of the "clean" type, that is to say it manifests an increase in viscosity of the cooled liquid stream.
Such a method is not suitable for substances of high purity, such as, for example, benzene, paraxylene, or orthoxylene, since these substances having a crystallization of the "clean" type, exhibit a random undercooling phenomenon, described in more detail in the description which follows, and this phenomenon is a handicap for the automatic continuous analysis of the crystallization point.